Halakhah for Niddah 70:16

And about this they, may their memory be blessed, said (Niddah 35b) that there is one 'wellspring' and the Torah made it impure, and the Torah [also] made it pure. [This is] meaning to say that the blood of birth that the Torah made impure and the blood of purification that the Torah made pure are from one place; and that is [that] both of them come out from the source (the uterus), which is a place of impurity. However the Torah made the matter depend on time and made impure the blood that comes from her at one time and made [it] pure at a second time. And blessed is the One that knows all wisdom, as He did every thing for a correct and necessary reason. And He, blessed be He, knows that up to a certain time, the blood is damaging; and [that] from then on, it is not damaging.

And the content of discharge is that which issues as a type of fluid from a man from the orifice of the member and he has no desire or pleasure from its emission. And they, may their memory be blessed, said (Niddah 35b) that it is similar to barley dough and, likewise, similar to the white of an addled egg; but semen is not like this, since it is similar to the white of an egg that is not addled. And that is the difference between them to distinguish between this and that.